This invention generally relates to communications systems, and particularly relates to controlling the output power and corresponding current consumption of a power amplifier.
Mobile wireless communications systems have numerous and oftentimes challenging design requirements not commonly shared with other types of communications equipment. For example, mobile terminals, such as cellular telephones, are expected to operate on battery power for significant intervals of time between re-charging. Battery life represents a key performance benchmark used by the consuming public to evaluate the relative desirability of available mobile terminals. Indeed, xe2x80x9ctalk timexe2x80x9d performance, which refers to the total length of time the mobile terminal will operate from a fully charged battery, is critical to many users. As designers continue reducing mobile terminal size and, consequently, battery capacity, commensurate reductions in mobile terminal power consumption are paramount.
Other challenges arise from the wireless communications standards themselves. IS-95, for example, is a Code-Division Multiple-Access (CDMA) 800 MHz digital cellular standard that requires mobile terminals to dynamically control their maximum transmit signal power over a defined range during operation, in accordance with changing conditions within the communications system. (Wide-band CDMA systems have similar power control requirements.) Transmit signal linearity must be maintained over the range of required transmit signal power. Both IS-95 and TIA/EIA-136 (an 800/1900 MHz Time-Division Multiple-Access digital cellular standard) require linear transmit signal amplification to meet strict signal fidelity requirements, and to avoid interference with other mobile terminals simultaneously active within the same service area.
Adjacent channel power ratio (ACPR) represents a key benchmark in evaluating transmitter performance and is commonly used to assess a mobile terminal""s potential for interference. Mobile terminals typically use some form of power amplifier for transmit signal amplification. To meet the aforementioned linearity requirements, transmit signal power amplifiers are commonly biased at an operating point providing linear amplification at the maximum required transmit signal power. This ensures transmit signal linearity when the mobile terminal operates at maximum transmit signal power, but results in relatively high levels of power amplifier quiescent current (bias current).
Such levels are power amplifier quiescent current are needlessly high when the mobile terminal is not required to operate at maximum transmit power. In light of the demanding battery life requirements imposed on mobile terminals, such inefficiency is particularly significant. Indeed, the transmitter in a typical mobile terminal represents a dominant component of operating current consumption, and thus represents a key area of concern in ensuring competitive xe2x80x9ctalk timexe2x80x9d performance ratings.
Reducing amplifier quiescent current as a function of output power represents one solution to this problem. Such schemes commonly employ a control voltage that varies amplifier quiescent current, as supplied by a bias network. This approach requires some form of analog control based on feeding back a signal proportionate to output power, which results in undesirable circuit complications. More importantly, varying the quiescent current of the power amplifier can result in degraded linearity, which results from changing the operating point(s) of the transistor(s) comprising the power amplifier. This can have serious implications for transmitted signal fidelity. Strict limits on ACPR allowed by standards such as IS-95 become a significant problem if linearity is compromised.
Another existing approach to quiescent current control entails using a switched-gain, multi-stage amplifier where the final stage of the amplifier is shut off and bypassed for low power operation. This action reduces the quiescent current of the amplifier at low power. This approach has several drawbacks, including large gain discontinuities and compromised linear output power adjustment.
Because overall amplifier gain depends on the final gain stage, switching it out for low power operation represents a substantial transmit signal gain discontinuity when changing from full power to low power operation. Also, because such approaches typically leave active only a driver stage in the low power mode, the available linear output power can be severely limited. A further drawback of the switched-stage approach is that switching out the final stage causes a large change in amplifier conduction angle, thus leaving this approach unsuitable for signaling applications requiring phase continuity to maintain the integrity of transmitted information.
Thus, there remains a need for a power amplifier that provides linear signal amplification over a range of selectable output power, while also providing relatively consistent signal gain and phase conduction angle over this range. Further, the needed power amplifier operates with a quiescent current proportionate to its selected output power, thereby reducing its quiescent current consumption at lower levels of output power.
The present invention provides both methods and apparatus for controlling the maximum output signal power and quiescent operating current of a power amplifier. A segmented power amplifier comprises two or more parallel amplifier segments that may be selectively enabled by a controlling system. When enabled, an amplifier segment provides an amplified output signal based on amplifying a common source signal received by the segmented power amplifier. These amplified source signals combine to form the final amplifier output signal from the segmented power amplifier. Overall amplifier quiescent current and maximum amplifier output signal power are both dependent upon the number of simultaneously enabled amplifier segments.
Exemplary embodiments of the segmented power amplifier of the present invention operate on radio frequency signals. Thus, a mobile terminal or other wireless communications device may advantageously use the segmented power amplifier of the present invention for transmit signal amplification. This allows the mobile terminal to transmit at different output power levels based on selectively enabling one or more amplifier segments in differing combinations. Because the quiescent current of the segmented power amplifier also depends upon the number of simultaneously enabled amplifier segments, the mobile terminal may reduce its operating current by enabling fewer amplifier segments in accordance with its transmit signal power requirements.
Preferably, each amplifier segment comprises a like number of one or more transistor amplification stages, with each stage enabled or disabled via a bias signal. A bias circuit, comprising a portion of the segmented power amplifier, selectively provides these bias signals to each amplifier segment in accordance with a bias control signal. Thus, a controlling system may disable all amplifier segments, or selectively enable one or more segments in a desired combination via the bias control signal. In exemplary embodiments, the bias circuit comprises a plurality of bias networks, with groups of bias networks associated with each amplifier segment. The bias control signal controls these groups of bias networks such that all amplification stages within a given amplifier segment may be commonly enabled or disabled.
In exemplary embodiments, the bias circuit biases all of the amplification stages in an amplifier segment either on or off (enabled or disabled), as detailed above, in response to the bias control signal. Preferably, when an amplifier segment is enabled, each amplification stage is biased to an operating point providing maximum linear signal amplification for that individual stage. Thus, when biased on, the overall amplifier segment provides linear signal amplification of the source signal. While each amplifier segment preferably provides similar signal gain, the segments may have substantially different levels of maximum linear output power. This allows a controlling system more flexibility in optimizing transmit signal power and quiescent current consumption, as given amplifier segments may be optimized for efficient operation at different levels of segment output power. Design of the individual transistor amplification stages comprising each segment provides this opportunity to set the maximum output power and quiescent current for each segment.
In exemplary embodiments, the segmented power amplifier of the present invention avoids large signal gain discontinuities between its possible combinations of enabled amplifier segments. That is, the segmented power amplifier of the present invention provides substantially the same signal gain whether one, all, or a combination of parallel amplifier segments are simultaneously enabled. Thus, while the maximum output power available from the segmented power amplifier of the present invention varies as a function of which parallel amplifier that simultaneously enabled, the signal gain does not vary appreciably. Further, the segmented power amplifier of the present invention additionally avoids significant changes in conduction angle between its different levels of maximum available output power. These benefits, as well as other advantages and features, will become apparent in the following detailed discussion.